1. Field of the Invention:
The present invention relates, broadly, to a low microinch surface generator adapted, particularly, for the manufacture of plastic contact lenses. Most specifically, the present invention relates to apparatus for the fabrication of soft or hydrophilic contact lenses by precision machining a lens precursor (e.g., button, blank or even bonnet) in the non-hydrated state.
2. Description of the Prior Art:
Numerous methods and apparatus are well known for the fabrication of optical surfaces on a variety of optically-efficient materials. Among these materials might be included various grades of glasses and plastics as well as, for reflective optical applications, metals. However, quantitatively, the manufacture of vision-corrective optical articles far outweighs the remaining areas of endeavor in this field. Surprisingly, therefore, it is found that few truly efficient methods and apparatus exist for the manufacture of vision-corrective optical articles; most approaches being rather pragmaticion an individual basis and possessed of anachronistic shortcomings.
Perhaps the routine use of obsolescent technology is most encountered in the manufacture of contact lenses for the correction of vision defects, and including the manufacture of the new, soft or hydrophilic polymeric contact lenses. With the modern shift from eyeglasses to contact lenses, the first generation hard synthetic plastic or glass-type contact lenses were initially fabricated based upon mere industrially-acceptable and conventional techniques. Thus, the hard plastic [typically polymethylmethacrylate or "MMA"] or glass lens precursors were formed in a rough state, ground, and subsequently polished either manually, or semimanually with the aid of conventionally employed optical polishing machines. Again, with the conversion from hard contact lenses to soft, hydrophilic lenses, antiquated methods and apparatus were perpetuated,notwithstanding the highly significant differing physical and chemical characteristics between these hydrophilic polymers and the materials for which the prior methods and apparatus were initially designated.
One deviation in the manufacture of soft contact lenses emerged in the form of the spin casting of the hydrophilic monomer during the very polymerization process therefor. While clearly a departure from conventional optical machining and polishing, the spin casting technique was found to be but a basically acceptable compromise, required primarily by the very nature of the lens material. Thus, the compromise is regarded as successful only inasmuch as the ease of process control has been fostered, but at the sufferance of optical quality and reproducibility. This is due to the fact that the anterior surface of the finished lens is predicated upon the shape and quality of the mold cavity, while that of the posterior surface is dictated by the centrifugal forces established during the spin casting process as the monomer polymerizes, viscosity, and the like. Because it is recognized that the surface of the eyeball is not uniform, but has a substantially varying rate of curvature generally corresponding to the apical portion of prolate ellipsoids, paraboloids, and hyperboloids, the ability to properly fit a centrifically cast hydrophilic contact lens with the optimum visual acuity is minimized. Moreover, even a centrifugally cast lens must be manually or otherwise edged. Accordingly, this technique has been found to be less than adequate in meeting the needs of the industry in properly balancing the ease of reproducibility and repeatability with the requirements of enhanced optical fit and power and, thus, wearer comfort and optical efficiency of the finished lens, particularly for those with astigmatic defects.
The art has recognized the advisability of producing methods and apparatus for machining or grinding the hydrophilic lens material in a non-swollen or dehydrated physical state. However, these approaches have not yielded a substantially improved finished lens for a number of reasons. Most significantly, the improvements in methods and apparatus heretofore proposed have merely centered about the modification of old technology, rather than an attempt to provide a totally new and improved system or concept which specifically accounts for the physical and chemical vagaries of the hydrophilic materials to be formed. Thus, it is routinely found that, for example, the tolerance limits of the machines employed far exceed those desirable tolerances for the finished product. Consequently, constant operator scrutiny and subsequent, costly rectifying procedures must be employed to yield a precision lens, or to otherwise salvage defective articles.
Furthermore, the very nature of the materials employed in the fabrication of these soft lenses mandates a critical appraisal of current production techniques. For example, in addition to all of the exacting operating procedures necessarily employed in the manufacture of high quality optical articles, the machining of hydrophilic polymers in a non-swollen or anhydrous condition entails process control far beyond that necessary for the analogous machining of glass or hard plastic lenses. For example, the hydration factor must be taken into account since the ultimate shape of the lens in the hydrated state may differ by 15%, or more, from that in the dehydrated stage. This further complicates the handling of the lenses during the fabrication steps since even a small amount of moisture, such as that on the tip of an operator's finger, or ambient humidity, can materially, locally swell the lens procursor. Consequently, should the operator touch the lens during the manufacture thereof, perspiration will cause local swelling which ultimately be machined or polished away during further process steps. When the lens then dehydrates at the local position, an obvious, and oftentimes fatal, flaw results, thus rendering the lens unsuitable for its intended purpose.
Yet other problems are encountered due to the nature of the physical and chemical characteristics and properties of soft contact lenses. For example, soft contact lenses not uncommonly have a greater diameter than the had lens counterparts. Also not uncommonly, a soft lens extends well into the scleral area of the eyeball, thus transgressing the sensitive limbus.
Moreover, due to the changing rate of curvature ofnot only the cornea but the scleral area, the optimum lens configuration will account for these differences and thus, be provided with a posterior surface which matches this changing rate of curvature of the cornea, jumps the limbus, and rests again on the sclera. And, while the scleral area is less sensitive than the cornea or limbus region, it is also essential that the edge radius of the lens be smooth and contoured to minimize eye irritation during wear. Also, while the posterior surface must account for the aspherical aberrations of the eyeball, the anterior surface must likewise be machined to very exacting tolerances, regardless of whether or not a plus or minus lens is to be yielded, to provide the required optical chaacteristics for the lens. To adequately account for the demanding designs inherent in quality optical contact lenses, it is thus essential to provide a maximum acceptable gross tolerance on the order of 0.001 inches, while optical surfaces should exhibit a finish of at least 4 microinches. Obviously, the greater the number of operating steps or points of human operator intervention, the less realistic become the attainment of these objectives.
Various automated processes, and apparatus therefor, have been proposed in the prior art. For example, U.S. Pat. No. 3,913,274 discloses a method and apparaus for making integrated multifocal lenses wherein a lens precursor is rotated in a lathe chuck and appropriately indexed in contact with a cutting tool or grinding wheel. The disclosed invention is predicated upon an adaptation of a conventional lathe whereby the lens is secured in a rotating spindle which also provides relative motion in two orthogonal directions in a plane perpendicular to the center of rotation of the lathe. The tool bit or grinding wheel is also caused to rotate about a variably controlled pivot point to allow for the cuttingor grinding of different curvature radii of the multifocal lens. Appropriate translation of the cutting tool and rotating lens is achieved by means of a digital computer.
While such apparatus are efficient for the manufacture of relatively large lenses, their utility is diminished when the workpiece is reduced to the much smaller size of a contact lens. For example, the column which supports the lens precursor, and which is tilted relative to the rotational axis of the lathe spindle, is not suitable for use as a fixture for supporting and rotating the much smaller contact lens. Moreover, the need to provide substantial superstructure in order to achieve sufficient relative freedom of motion tends to decrease dimensional stability by increasing the number of sources which contribute to dimensional error. Also, it is obvious that significant operator intervention is needed in order to practice the disclosed process, further contributing to potential sources of dimensional instability and lack of reproducibility from lens to lens.
Another apparatus is disclosed in U.S. Pat. No. 3,835,588, relating to a lenticular contact lens lathe. Again, because the apparatus is patterned on a standard contact lens lathe, which has been modified to provide for an orthogonal translation system via cascaded movable carriages, inherent dimensional instability is built within the system. Moreover, it is necessary to cast or otherwise preform the lens precursor with the posterior surface thereof. Consequently, the same disadvantages obtaining with the spin casting of hydrophilic monomers is indigenous to that disclosed process.
Similar apparatus and processes are disclosed in the U.S. Pat. Nos. 3,064,531 and 3,100,955, wherein the lens precursor must first be subjected to a substantial preforming operation in order to render the same compatible with a lathe chuck or other conventional securing member. In the case of the former patent, the lens precursor is threaded for insertion within a special chuck having a matting thread. In the case of the latter, the precursor is first formed with a peripheral ear for restraint within a sleeve. Obviously, the preforming steps are highly undesirable.
In an effort to minimize operator intervention by maximizing the number of process steps on a lens blank between mounting and demounting thereof, a quite mechanically exotic apparatus is disclosed in U.S. Pat. No. 3,686,796. The machine therein described performs multiple operations, including machining, lapping, edging, and/or polishing a lens which is retained in a rotatable lens holder relatively indexable with respect to a plurality of pivotally mounted spindle heads, each for performing a given opeation. Obviously, the complexity of such a machine and the need to provide the great number of separate machine tools which must be accurately registered fromn step-to-step are highly undesirable from a commercial point of view.
Conventional pantographs and cam followers have been adapted for fabricating contact lenses, but not without suffering many of the problems noted above and without providing the ability to produce high quality articles in reproducible, commercially-acceptable quantities. These deficiencies may be attributed to, for example, the complexity of mechanical linkage, inherent machine and ambient vibrations, the inability to produce an article of better quality than that of the pattern's surface, etc.
Yet a further problem evident with prior art methods and apparatus for forming contact lenses is the inability of the same to yield an edge, as machined, without defects. Consequently, various postforming polishing operations such as those disclosed in U.S. Pat. Nos. 3,032,936 and 3,736,115, are necessary. Again, by increasing the number of operations, potential additional sources of error are encountered.
Accordingly, the need exists to provide a scientifically sound concept, method and apparatus for the reproducible, simple, and efficient manufacture of high quality optical surfaces on an optical lense precursor, whereby the number of process steps are minimized and which substantially diminishes the need for human intervention.